The present invention relates to a system for processing dross which forms on a molten metal bath during melting and holding in a furnace and more particularly to a system for reclaiming metal entrapped in the dross.
Various techniques have been developed for reclaiming metals from drosses to reduce metal losses associated with melting and holding molten metal in a furnace. One technique has been to transport the dross to a site remote from the furnace site for processing. A second technique processes the dross at the furnace site. These techniques frequently employ a reaction vessel where the dross is stirred or agitated to promote separation of the metal entrapped therein. During processing, the entrapped metal forms a pool of molten metal at the bottom of the reaction vessel. The molten metal is then drained from the bottom of the reaction vessel into a collection vessel, and the spent dross is dumped from the reaction vessel.
U.S. Pat. No. 3,198,505 illustrates an apparatus used for remote processing of the dross. This apparatus includes a frame to which a motor-driven stirrer is mounted. A drum is employed as a reaction vessel for containing the dross. The reaction vessel is designed to be transported by forklift from the furnace site, where the dross is loaded into the reaction vessel, to a remote site where the frame is located. A movable platform is provided to raise the drum so as to immerse the stirrer into the dross in the drum. A recovery tray resides below the platform to collect the drained metal. This apparatus requires dedicated space, making it impractical for use where free space to accommodate the apparatus is not available. The remote processing of the dross also introduces temperature losses during transport of the dross and can complicate the effective processing of the dross.
U.S. Pat. No. 4,121,810 illustrates an in situ apparatus which can be effectively used for processing larger volumes of dross. The ""810 patent discloses a container with a motorized stirrer which extends through the bottom of the container. The container is mounted on a wheeled carriage to allow positioning it near a furnace for loading dross, thus avoiding the need for a dedicated space. Trunnions are provided to allow pivoting the container to dump the remaining dross after the metal has been drained. A frusto-conical shield in the bottom of the container protects the bearing of the stirrer drive shaft from the dross and the molten metal. The drive shaft is driven via a chain by a motor, which is apparently mounted to the side of the container to allow the container to be tipped without disengaging the drive mechanism. The requirements of shielding the drive bearing and mounting the motor to the container complicate the structure of the container, and make replacement of such a container impractical. While the system of the ""810 patent is mobile, it is complex, making it difficult to maintain, and makes the elimination of the spent dross difficult.
U.S. patent application Ser. No. 09/233,564, now issued as U.S. Pat. No. 6,136,262, the inventor has disclosed and claimed an apparatus suitable for in situ processing which provides a mobile apparatus with reduced complexity; however, such does not provide for automated stirring, which effectively limits the quantity of dross that can be conveniently processed since it relies on the operator stirring the dross. Furthermore, the effectiveness of such apparatus is dependent on the skills of the operator. Also, for larger volumes of dross, the operator is required to stir the dross for a substantial time and may choose to foreshorten the cycle since, while stirring, the operator is working in close proximity to the hot dross and is exposed to fumes which are generated as the dross reacts with exothermic compounds and fluxes employed to enhance the recovery of the entrapped metal.
Thus, there is a need for a dross processing system which avoids the disadvantages of the devices discussed above and which is simple to operate, requires minimum maintenance, and yet provides a high recovery rate.
The present invention is for an in situ dross processing system for reclaiming entrapped metal from a dross. The in situ dross processing system of the present invention reclaims entrapped metal from the dross which develops while melting metal and holding the molten metal in a furnace. The system of the present invention has been found to have particular utility in processing the dross associated with molten aluminum alloys.
In an elementary form, the dross processing system has a substantially vertical support and means for positioning the support in close proximity to the furnace. The substantially vertical support terminates in an upper support region and a lower support region. In one embodiment, the means for positioning the support is a bracket affixed to the furnace and engaging the lower support region, while in another embodiment a stand is attached to the lower support region and serves to position the substantially vertical support in close proximity to the furnace. The stand can be designed to either anchor the substantially vertical support with respect to the furnace or can be mounted on wheels to facilitate the movement of the dross processing system to and from a position in close proximity to the furnace.
A reaction vessel is provided, into which the dross is loaded and processed to extract molten metal entrapped therein and separate it therefrom, by allowing the molten metal to settle to the bottom of the reaction vessel. The reaction vessel preferably has a substantially vertical sidewall, terminating in an upper rim and a lower rim, and a bottom member attached to the lower rim so as to form a concave surface when viewed from the sidewall. Some limited degree of draft is preferred in the substantially vertical sidewall to better facilitate dumping the contents of the reaction vessel. Furthermore, having draft in the substantially vertical sidewall, in combination with the bottom member having a concave surface promotes relief of the resulting stress field from thermal expansion by axial symmetric strains and prevents local buckling even when the reaction vessel is fabricated of relatively thin stock.
A port is located in the bottom of the reaction vessel and is preferably formed by a cylindrical sleeve passing through the bottom member. The port is configured to be sealingly engaged by a port plug, which is preferably fabricated from a refractory fiber material such as an alumino silicate that is packed into the port to seal the port and avoid run-out of the molten metal collected in the bottom of the reaction vessel. It is further preferred that a removable plug support plate be positioned below the sleeve and maintained in position by a plate support rack attached to the bottom of the reaction vessel. The plug support plate provides additional support to the plug to withstand the pressure from the head of recovered metal which collects in the bottom of the reaction vessel. It is further preferred that the plate support rack have two substantially horizontal members which attach to the reaction vessel and on which the support a plate slidably rests. In a preferred embodiment, the two substantially horizontal members are formed as U-shaped plate support bars on which the plug support plate rests.
The dross processing system may be configured such that the reaction vessel can reside over the furnace for loading the dross. In such cases, it is preferred that the reaction vessel be fabricated of metal sheet stock. For processing dross from aluminum alloys, a low alloy stainless steel such as 304 stainless steel has been found adequate for forming the metal reaction vessel. For ease in fabrication, it is preferred that the reaction vessel be fabricated from a multiple part assembly having a substantially vertical sidewall, which can be readily formed from metal sheet stock, to which a bottom member is attached. The bottom member can be readily formed by spinning metal sheet stock into a dish shape which is then welded to the substantially vertical sidewall. The spun metal sheet stock is provided with a hole, into which is welded a cylindrical sleeve which serves as a port. While a 304 stainless steel is adequate for processing drosses of aluminum alloys when the reaction vessel is so fabricated, it is preferred that a higher alloy stainless steel such as 310 stainless steel be employed for the bottom member and the substantially vertical sidewall. However, in any case, it has been found practical to form the cylindrical sleeve from low carbon steel. It has been found that the substantially vertical sidewall, the bottom member, and the cylindrical sleeve can be effectively joined by MIG welding with a filler metal selected to avoid sensitization of the weld and heat-affected zone during fabrication and use.
ER309 stainless steel has been found adequate as such a filler metal, but ER310 stainless steel is preferred.
The use of a metal reaction vessel allows the reaction vessel to be preheated when residing above the furnace by the radiant energy from the molten metal contained therein. To optimize such pre-heating, the reaction vessel preferably has minimal thermal mass. To decrease the thermal mass while maintaining the structural integrity of the reaction vessel, a thin gauge metal can be used for the reaction vessel, reinforced by an expanded metal sheet tack welded to the outside of the reaction vessel.
For applications where the dross processing system has the reaction vessel at all times positioned outside the footprint of the furnace, a dual shell reaction vessel is preferred. Preferably, this reaction vessel is fabricated from metal sheet stock and has an inner substantially vertical sidewall spaced apart from an outer substantially vertical sidewall, both of which attach to a common upper rim. The inner substantially vertical sidewall terminates in an inner lower rim which is separate and spaced apart from an outer lower rim which terminates the outer substantially vertical sidewall. For such a reaction vessel, again an inner bottom member can be spun from metal sheet stock and welded to the inner lower rim to form an inner shell. Similarly, an outer bottom member can be spun from metal sheet stock and welded to the outer lower rim, which is positioned with respect to the inner lower rim such as to maintain a spaced-apart relationship between the inner bottom member and the outer bottom member, thus forming an outer shell. In the dual shell reaction vessel, a sleeve passes through both the inner bottom member and the outer bottom member, and serves as a port through which reclaimed metal from the dross is drained. The sleeve also provides additional structural rigidity to the reaction vessel. Greater strength may be provided by placing a refractory material such as Fiberfrax Blanket refractory fibers between the inner shell and the outer shell.
When the dross processing system is not designed to have the reaction vessel reside over the molten metal, a dual shell reaction vessel can also be fabricated having an inner shell fabricated from metal sheet stock, which has an expanded metal framework attached thereto, and an outer shell which is fabricated from refractory materials applied over the expanded metal framework. The use of refractory materials reduces heat loss from the dross held therein without providing a second metal shell which is spaced apart from the first shell.
For the embodiments of the in situ dross processing system of the present invention, a reaction vessel support mounts the reaction vessel with respect to the substantially vertical support. The reaction vessel is mounted to the reaction vessel support by a reaction vessel mount, which can be an integral part of the reaction vessel support and which pivotably attaches the reaction vessel to the reaction vessel support. Means are provided for retaining the reaction vessel in a horizontal position, where the upper rim of the reaction vessel is substantially horizontal. Means to pivot the reaction vessel are provided to pivot the reaction vessel to a dump position where the contents of the reaction vessel are eliminated. Mechanical means for such can be provided by a linear actuator attached between the reaction vessel and the reaction vessel support, by a counterweight system in combination with hand raising, by a motor in combination with appropriate gearing, or by configuring the reaction vessel mount such that the reaction vessel pivots to the dump position under the influence of gravity. When the substantially vertical support is affixed with respect to the furnace or cannot be readily moved with respect to the furnace and the reaction vessel resides over the furnace, the reaction vessel support is rotatably mounted with respect to the furnace about a vertical axis to allow the reaction vessel to be swung away from the furnace for dumping. When swung away from the furnace, the reaction vessel resides outside the footprint of the furnace, making it accessible to the user so that its contents can be readily eliminated, and the reaction vessel inspected and made ready for its next cycle. In such instances it is also preferred that the reaction vessel support can be raised and lowered such that its height can be adjusted so as to allow the reaction vessel to be readily swung to a position outside the footprint of the furnace and, when so positioned, lowered to facilitate loading of the vessel.
A motor having a drive shaft with a free end is connected to a motor arm which in turn is mounted with respect to the substantially vertical support such that the motor is positionable with respect to the reaction vessel. The motor is raisable and lowerable with respect to the reaction vessel. The motor is preferably also rotatable to a position where a footprint of the motor does not project onto the reaction vessel. This latter motion is advantageous in allowing the motor to be swung away from the reaction vessel to assist in the loading of the reaction vessel with dross to be processed.
A motor mount, which can be an integral part of the motor arm, secures the motor with respect to the motor arm such that the drive shaft is positioned in a substantially vertical orientation with the free end of the drive shaft directed toward the reaction vessel when the motor resides above the reaction vessel.
The dross processing system is provided with means for displacing the motor with respect to the reaction vessel, thus moving the motor between a raised position and at least a first lowered position which places the motor into relatively close proximity to the reaction vessel. When the means for displacing the motor provide translational motion of the motor, typical examples of such means include the use of linear actuators, rack and pinion gearing, and counterweight systems. Alternatively, the means for displacing the motor can provide a pivotable motion of the motor with respect to the reaction vessel.
When the means for displacing the motor provide translational motion of the motor, it is further preferred that means for securing the motor in the at least a first lowered position and in the raised position be provided. When a linear actuator is employed as the means displacing the motor, the linear actuator can be configured to cycle from a full extension position to a minimum extension position and can serve the dual functions of positioning and securing the motor. When a counterweight system is employed to balance the weight of the motor and associated structure and the motor is manually raised and lowered, a slot and key can be used to guide the movement of the motor and lock it at the appropriate locations.
A stirrer is provided to stir the dross and promote the reaction of the dross with an exothermic compound which is added to the dross to raise the temperature of the dross and promote the coalescence of the metal entrapped therein. The stirrer attaches to the free end of the drive shaft and is mounted such that the stirrer is positioned below the upper rim of the reaction vessel when the motor is in its at least a first lowered position, and is positioned outside the reaction vessel when the motor is in its raised position.
It is further preferred that the stirrer be an impeller with a plurality of blades to promote the mixing of the exothermic compound added. The blades can be radially arranged around the drive shaft and extend substantially normally thereto, or alternatively the blades can be mounted on a hub which is normal to the drive shaft, and be arranged in a spaced-apart relationship to each other and in a non-intersecting relationship with the axis of the drive shaft.
It is also preferred for a protective shield to be provided, which is configured to engage the upper rim of the reaction vessel when the motor is in the at least a first lowered position to maintain fumes generated by the reaction of the exothermic with the dross within the confines of the reaction vessel. The protective shield also serves to reduce heat loss from radiation and any splashing of the dross and entrapped metal as the dross is stirred. The protective shield is provided with a drive shaft passage through which the drive shaft of the motor passes.
In another preferred embodiment, means for providing lateral movement of the stirrer in the vessel are provided, which reduces the size of the impeller needed to stir the dross and promote the reaction of the dross with an exothermic compound added thereto.
For embodiments where the stand is mounted on wheels to allow the dross processing system to be readily transported to and from the furnace area and where the reaction vessel will not be positioned over the molten metal bath, it is preferred that the stand also have a platform onto which a spent dross container can be placed. The spent dross container preferably has pivotally and lockably attached thereto a pair of diametrically opposed fork receptors for engagement by the forks of a fork lift to facilitate moving the spent dross container and dumping the spent dross therefrom. Such a spent dross container is also well suited for use with embodiments where the dross processing system is stationary.